Clinical correlative, molecular genetic and pharmacological data show certain tumours are dependent on ERK5 signalling, and that ERK5 inhibition may therefore result in antitumour activity.
Clinical Correlative Data
ERK5 is an independent prognostic biomarker in prostate cancer. Cytoplasmic ERK5 staining correlates with Gleason sum score, bony metastasis and locally advanced disease. Furthermore, nuclear ERK5 localisation is associated with poor outcome and recurrence in castrate-resistant prostate cancer. Thus strong nuclear ERK5 localisation is an independent prognostic factor (p=<0.0001). In matched tumour samples (n=26), taken before and after the development of resistance to anti-androgen therapy, ERK5 nuclear expression is associated with relapse (p=0.008). A similar study in breast cancer (84 cases) shows that ERK5 is over-expressed in 20% of patients and that, as in prostate cancer, expression is an independent prognostic biomarker for reduced disease-free survival. In liver cancer ERK5 over-expression has been observed in 11 of 43 samples, associated with modest gene amplification in a subset of patients, and in oral cancer elevated phospho-ERK5, but not phospho-ERK1, is associated with lymph node metastasis.
Pharmacological Studies
Critical pre-clinical data that suggest the viability of ERK5 as a small molecule drug target come from recent studies with the prototype inhibitor XMD8-92. The compound was shown to have modest anti-proliferative activity in tumour cell lines and endothelial cells, via effects on ERK5-mediated promyelocytic leukaemia protein phosphorylation and p21. More importantly, XMD8-92 was well tolerated in mice, inhibited tumour ERK5 and produced pronounced retardation of both human and murine tumour growth, i.e. complete and immediate cessation of tumour growth on commencement of treatment, with no toxicity. Furthermore, in a matrigel plug assay for bFGF-induced angiogenesis, XMD8-92 completely abrogated blood vessel formation, consistent with the role of ERK5 signalling in endothelial cell biology.
Molecular Genetic Studies
An increasing body of mechanistic data indicates that ERK5 plays at least three key roles in tumour biology, i.e. in cell proliferation and survival, in invasion and metastasis, and in angiogenesis.
Proliferation and Survival
The first role of ERK5 in tumour biology relates to mitogenic and survival signalling in tumour cells where ERK5 is downstream of cell surface tyrosine kinase receptors; the epidermal growth factor/HER receptor family being most frequently implicated. Dominant-negative ERK5 constructs are growth inhibitory in a number of cell lines, although not in every case, and when implanted into immune-deprived mice ERK5 over-expressing human prostate cancer cells result in more aggressive tumour growth when compared to non-transfected cells. Importantly, ERK5 is a downstream kinase and hence ERK5 inhibitors will not be subject to the limitations of growth factor receptor-targeted therapies, such as receptor tyrosine kinase inhibitors or receptor blocking antibodies, where RAS and RAF mutations abrogate activity; ERK5 inhibitors may retain activity in the face of RAS/RAF mutations.
With regard to survival signalling, ERK5 over-expression protects cells from, and dominant negative ERK5 sensitises cells to, apoptosis induced by physical, cytokine and therapeutic drug stimuli; however, as with many targeted drugs, the effect of ERK5 on tumour cell proliferation and survival is cell type specific.
Invasion and Metastasis
Compelling and unequivocal data demonstrates that ERK5 signalling promotes cell migration and invasion, a property that is consistent with the clinical relationship between tumour ERK5 over-expression and the presence of metastatic disease. For example, an orthotopic model of prostate cancer, demonstrates that ERK5 promotes the formation of metastases. Similarly, in breast cancer cells ERK5 knockdown reduces hepatocyte growth factor-stimulated migration, and in an in vivo orthotopic breast cancer model ERK5 knockdown results in a marked reduction in lymph node metastasis. Lastly in osteosarcoma, where ERK5 can be over-expressed, ERK5 knockdown results in a pronounced inhibition of invasion, associated with reduced MMP9 expression.
Angiogenesis
ERK5 inhibitors may also inhibit tumour angiogenesis. A substantial body of data based initially on constitutive and conditional knockout mouse models show that ERK5 is required for endothelial cell survival, in particular maintenance of vascular integrity and endothelial cell migration. Both constitutive MEK5, the obligate kinase required for ERK5 activation, and ERK5 knockout mouse embryos die at day 10.5 from tissue failure that includes impaired cardiac development. Importantly, in conditional ERK5 knockout mice ERK5 ablation in adults disrupts vascular integrity and results in death from multiple organ haemorrhage, consistent with a key role for ERK5 in endothelial cell biology. Further studies unequivocally link ERK5 signalling to tumour angiogenesis in two in vivo mouse tumour models, and hence treatment with an ERK5 inhibitor is expected to have profound therapeutic effects on tumour vasculature in solid cancers.
Therefore there is a need for the development of selective ERK5 inhibitors for the treatment of diseases.